1. Field of the Invention
The present invention relates to electrically conductive thin films. In particular, the present invention concerns a process for producing transition metal and transition metal carbide thin films by a CVD type process using boron compounds as reducing agents.
2. Description of Related Art
In the deposition of metal and metal carbide thin films by CVD, a reducing agent is utilized for lowering the oxidation state, i.e. reducing the metal source compound to a reduced form.
Various reducing agents have been tested, for example cadmium metal vapour or zinc metal, has been proposed previously. The problem of the first reducing process is that cadmium is a rather toxic heavy metal. Zinc, on the other hand, is an efficient reducing element but it may cause troubles in the manufacturing of integrated circuits due to its high diffisivity.
Most efficient reducing elements are found in group 1 of the periodic table of elements. Heavy alkali metals, such as sodium and potassium, can be vaporized easily but they form low-volatility metal halides. Thus, the processing temperature of metal films by the reduction of metal halides with e.g. sodium or potassium must be kept rather high (preferably above 600xc2x0 C.) to be able to evaporate alkali halides away from the substrate surface.
The reducing agents most typically used at present are hydrogen and silanes (SixHy, where x and y are integers).
Typically, the metal source compound used is a halide of the desired metal. When the metal is reduced, the halogen bound to the metal is released and often reacts with the hydrogen or hydrogen-containing reducing agents, provided that hydrogen in the reducing agent is bonded so weakly that it readily reacts with other compounds. Problems may arise, if a hydrogen halide is formed. Hydrogen halides, such as BF and HCl typically deteriorate the surface of the substrate or the thin films layers previously formed.
Silanes that have only silicon and hydrogen in the molecule are extremely reactive compounds and set high requirements for the operation of a process involving the use thereof.
An example of a process in which H2 and SiH4 are used as reducing agents and WF6 is used as the metal source material for depositing elemental metal thin films is disclosed by M. F. Bain. (Deposition of tungsten by plasma enhanced chemical vapour deposition, J. Phys. IV, 9 (1999) pp. 827-833). It is stated in the publication that the deposition of tungsten metal from WF6 on titanium surface caused the formation of titanium fluoride species which incorporated at the titanium-tungsten interface. High fluoride content at the interface may lead to poor adhesion between the layers.
Further, according to literature, hydrogen looses reactivity at low substrate temperatures. Additional energy in the form of e.g. plasma was needed in order to be able to use hydrogen for the reduction of transition metal halides. It is easily understood that the use of extra equipment, such as a plasma generator, adds cost to the process.
The use of other reducing agents, has been disclosed in, for example, JP 6069157, which relates to a CVD process where titanium halide gas is reduced by germane (GexHy) into titanium metal. Germanes have also been used for reducing WF6 into W metal. It must be noted that germanes are less stable thermally than silanes so that the reduction process is probably more complicated with germanes. There have been indications of Ge impurities in the films in the form of a W3Ge compound.
U.S. Pat. No. 5,946,598 discloses a process in which tungsten fluoride is reduced with silicon on a substrate surface. First, elemental silicon is deposited on substrate. After that tungsten fluoride and silane are introduced into the reaction space. Silicon surface reacts with tungsten fluoride leaving tungsten metal in place of silicon. It must be noted that the applicability of the process can be limited because it is rather difficult to form conformal silicon coating on deep trenches and vias of damascene structures.
A. Ludviksson has disclosed a CVD process where trimethylamine alane reduces titanium halides (Low-temperature thermal CVD of Tixe2x80x94Al metal film using a strong reducing agent, Chem. Vap. Deposition, 4 (1998) pp. 129-132). There were some troubles with halide contamination of the metal film when aluminum was not codeposited.
WO9851838 discloses a CVD process of growing W metal with B2H6 as a reducing agent. The claims in the patent cover group IIIA and IVA hydrides. This is a multiple-step process which includes a nucleation step using silane as a reducing agent.
Triethyl boron (shortened as TEB) has been used by A. Y. Polyakov for the deposition of Ga1xe2x88x92XBXN thin films at the substrate temperature of 450-1000xc2x0 C. (Growth of GaBN Ternary Solutions by Organometallic Vapor Phase Epitaxy, Journal of Electronic Materials, 26 (1997) pp. 237-242). In this publication boron was incorporated into the growing t film. The deposition temperature is rather high. Our laboratory experiments show that TEB partially decomposed on the substrate above 400xc2x0 C. leaving a residue when TEB was carried to the reaction space in flowing nitrogen atmosphere that had an absolute pressure of 5-10 mbar. The residue from TEB possibly consisted of hydrogen-rich boron carbide.
It is an object of the present invention to eliminate the problems of the prior art and to provide a novel method for growing metal and metal carbide thin films by a Chemical Vapour Deposition (CVD) type process.
In the present context, electrically highly conductive transition metals and transition metal carbides are defined as metalloids.
The invention is based on the surprising finding that a metalloid thin film can be grown on a substrate by a chemical vapour deposition type process by feeding to the reaction space a metal source material and a novel reducing agent in vaporised state. The reducing agent is capable of reducing said metal source material into reduced state at the conditions in the reaction space. The compound used as the reducing agent is selected from the group of boron compounds, said compounds having at least one boron-carbon bond.
More precisely, the present process is characterised by what is stated in the characterising part of claim 1.
A number of considerable advantages are obtained by means of the present invention. The thin film obtained by the present process has less impurities than those prepared by conventional processes. The use of particular reducing agents enhances the purity of the obtained film, since the compounds used in the present invention form volatile reaction by-products.
With the aid of the present process a metalloid thin film can be grown at temperatures below 400xc2x0 C. The use of low temperatures is beneficial especially for making metal and metal carbide layers for damascene structures in integrated circuits, and, of course means savings in energy costs and total processing time. Low processing temperature also means that there is less mechanical stress between the thin film and the substrate
With the aid of the present process, metalloid thin films can be grown very fast and the formation of free hydrogen halides is avoided.
The grown metal thin films can be used as electron conductors or interconnects or seed layers in integrated circuits. The grown metal carbide thin films can be used as diffusion barriers, for example between a dielectric, in particular one having a low k-value, and a copper layer. Diffusion barrier layers preferably comprise carbides of tungsten (W), tantalum (Ta), hafnium (Hf), zirconium (Zr) and/or titanium (Ti), more preferably tungsten carbides. Noble metal thin films, preferably comprising gold (Au), palladium (Pd), platinum (Pt), rhodium (Rh) and/or iridium (fr), also find use as protective layers on parts which in demanding environments.
Next, the invention will be more closely examined with the aid of the following detailed description and by making reference to the attached drawings.